I would define "stupid light" as compromising your health or safety in order to save weight. As others said, it can be simply defying common sense just to get a lighter base weight.
I think it is most important with items on the classic 10 essentials list and selecting items that aren't suitable for the conditions. Skurka's example of bug protection certainly hits the common sense side.
Trying to reduce base weight by leaving out core items like rain gear, first aid kit, compass and map, etc is stupid light to me. Extend that to using poor substitutes like a button compass for primary navigation, a single edge razor blade rather than a basic knife, or a tiny button cell LED for primary lighting. Taking toys just to say you checked off the essentials is fooling yourself IMHO. That doesn't mean loading up on heavy stuff, but a 2oz compass isn't going to bloat your spreadsheet.
To me, the down/synthetic debate is more an issue of local climate and conditions rather than not bringing good insulation at all. I've made the point that taking a thin puffy can be a waste of weight if you end up too cold in camp. Likewise the preference in sleeping pad. The stuff you bring needs to work.






